Lots of $$$

Well instead of guessing you could actually do the math.

 

Say you have a 500 watt power supply. That means that the maximum power you can draw is 500 watts at any time. 

 

If you are constantly drawing 500 watts (hypothetically):

 

You will pay around .10 usd per kwh for electricity, give or take a few cents. 

(500 watt power supply / 1000) = .5 kw

If you draw .5kw for one hour, it will cost you (.5 kw * .10 usd / kwh) = .05 (usd / hour)

 

This means that if you have a 500 watt power supply, the MAX that you can spend is 5 cents usd per hour while running your computer. 
On average, you will likely spend a fraction of this, maybe half.

 

tl;dr:

500 watt power supply max cost is 5 cents usd per hour.

 

Using: http://www.paystolivegreen.com/energy-savings-calculator/ to run my laptop for 24 hours its like 20 cents. So It shouldn't be too much for u truthfully.